That’s just weird US notation – some people use an asymmetrical symbol also for non-polarized caps, while the ones that are polarized are marked with a + (e.g. C3 in that schematic). Other hints here are where they are in the circuit, and their value & unit prefix.
(TBH I’m not entirely sure why he bothers with caps here, I at first thought it was to deal with potentially noisy external CV but there’s no external CV here. If it’s to avoid the wiring picking up noise you want them closer the chip, not on the front panel.)
Voltage divider for pin 3. Using 100k resistors instead of 10k. Because voltage divider would still work the same?
The analog PIC/AVR inputs are a bit special; while they measure voltage, they need a certain amount of current to charge an internal capacitor, and datasheets recommend staying at 10k or below:
But none of this explains why you’re frying regulators.